BEIJING — Mao Zedong famously dismissed the atomic bomb as a “paper tiger,” able to kill and terrify, but not decisive in war. Even so, China built a nuclear arsenal of its own, and now concerns about the effectiveness of that arsenal as a deterrent are driving it into confrontation with the United States over an antimissile system being built in South Korea. Here’s an explanation of why.

How big is China’s nuclear arsenal?

China conducted its first nuclear test in 1964, and has developed a stable of nuclear missiles. But it is not a big stable, compared with the thousands of warheads held by the United States and Russia.

China does not reveal the size of its nuclear forces. It has about 260 nuclear warheads that could be put on missiles, and by the Pentagon’s latest estimate, China has between 75 and 100 intercontinental ballistic missiles. Some estimates are lower, and one recent assessment said 40 to 50 of China’s ballistic missiles could reach the continental United States.

The United States has deployed about 1,370 nuclear warheads and has stockpiled more than 6,500, and has submarines and aircraft able to launch nuclear weapons.

China has also built several submarines that can launch nuclear missiles. But even its latest-model submarine “is noisy and quite vulnerable to anti-submarine warfare,” and therefore is not a very potent addition to its nuclear deterrent, M. Taylor Fravel, a professor at the Massachusetts Institute of Technology, and Fiona S. Cunningham, a graduate student there, who recently published an assessment of China’s nuclear modernization, said by email.

China has also been upgrading some of its missiles so that several nuclear warheads can be placed on a single missile that then unleashes them on different targets.

China has had the ability to put multiple warheads on missiles since the 1990s, but seems to have done so only recently, when some missiles were installed with three or four warheads, said Jeffrey Lewis, an expert on China’s nuclear forces at the Middlebury Institute of International Studies. This showed how China has been cautious in playing catch-up to the United States, he said.

“I don’t think that the Chinese and U.S. have, historically, experienced the kind of tit-for-tat modernization that we saw during the U.S.-Soviet arms race,” Mr. Lewis said. “The Chinese more or less modernized for their own reasons and according to their own ideas.”

Why has China’s nuclear arsenal stayed relatively small?

By the time China joined the nuclear club, the United States and Russia were already well ahead in building a stockpile of weapons. Mao decided to stick to a relatively small arsenal big enough to serve as a deterrent, and that decision was made a fait accompli by the political turmoil of Mao’s era, which held back the nuclear weapons program.

“China’s leaders thought that the important thing was to master the technology,” Mr. Lewis said. “While the United States did fine calculations of the deterrence balance, Chinese leaders tended to think of deterrence like a checklist of achievements.”

Ever since, Chinese nuclear doctrine has stuck to the idea of a “minimum means of reprisal,” with a force designed to survive and retaliate after an initial nuclear attack. Alongside that, China has a nuclear “no first use” policy: that it will not be the first to launch nuclear weapons against another nuclear foe, and that it will not use its nuclear weapons against a country without nuclear weapons.

Even so, China has been expanding and upgrading its nuclear forces, and that modernization may speed up if the government feels that it is falling too far behind the United States.

“China is probably confident in its ability to be able to retaliate, but given the size and sophistication of U.S. nuclear forces and the steady development of ballistic missile defenses, coupled with China’s small nuclear arsenal, the margin for error is thin,” Mr. Fravel and Ms. Cunningham said.

Why does China fear the antimissile system?

The Chinese government worries that the American antimissile system, called the Terminal High Altitude Area Defense, or Thaad, could erode its nuclear deterrent — its ability to scare off potential foes from ever considering a nuclear attack.

Its chief worry is not that Thaad could take down missiles: the system offers a canopy of potential protection over South Korea, but does not have the reach to bring down China’s intercontinental ballistic missiles. Instead, China’s complaint is focused on Thaad’s radar system, which Chinese experts have said could be used to track the People’s Liberation Army’s missile forces.

Deploying Thaad’s current radar system “would undermine China’s nuclear deterrence by collecting important data on Chinese nuclear warheads,” Li Bin, a nuclear weapons expert at Tsinghua University in Beijing, wrote last week.

He and other Chinese experts say the radar could identify which Chinese missiles are carrying decoy warheads intended to outfox foes. That would be like being able to see what cards China holds in a nuclear poker game, and that could weaken China’s deterrent, they say.

“For China this is a very important point, because its missiles are limited in number to begin with,” Wu Riqiang, a nuclear expert at Renmin University in Beijing. That meant, he said, “China could lose its nuclear retaliatory capacity.”

For China, it does not matter that the American and South Korean governments have said Thaad is meant only to foil North Korean missiles. Mr. Wu said.

“What we worry about is the ability. It doesn’t matter to us whether the United States says this is aimed at North Korea or China,” Mr. Wu said. “If there’s this ability, then China must worry.”

Are China’s fears justified?

Chinese experts are nearly unanimous in supporting Beijing’s criticisms. But quite a few foreign experts say those fears are overstated or unfounded. The United States already has access to radar systems in Qatar and Taiwan able to peer at China’s missile tests, and Japan has two radar systems just like the one used for Thaad, Mr. Lewis said.

“I don’t see the deployment of Thaad in South Korea as a significant improvement in the ability of the U.S. to monitor Chinese missile tests,” he said.

The Chinese government appears to have an exaggerated view of the Thaad radar’s abilities, two experts, Jaganath Sankaran and Bryan L. Fearey, wrote in a recent paper. That radar is often said to have a range of about 620 miles. Some Chinese experts say its reach could be much farther.

But in practice the range could be much lower and “not possess the ability to track Chinese strategic missile warheads/decoys,” Mr. Sankaran and Mr. Fearey wrote. “The Thaad radar simply cannot cover the entire or even a substantial part of the Chinese mainland.”

Even so, China’s real, underlying worry appears to be that Thaad could open the door to a much wider, more advanced fence of antimissile systems arrayed around it by America’s allies, several experts said. That would magnify Chinese worries about the effectiveness of its nuclear deterrent, and entrench Chinese fears of encirclement by a coalition knit together by a shared antimissile system.

“I think this is what really worries them, because then what you have is the basis for a common interoperable system,” said Michael J. Green, the former senior director for Asia in the National Security Council under President George W. Bush. “I think it’s more about the creation of a virtual collective security system,” he said of China’s worries about Thaad.

Will China alter its nuclear policy in response to Thaad?

Last week, the Global Times, a stridently nationalist Chinese newspaper, warned in an editorial that China could consider abandoning its “no first use” policy if Thaad leads to other antimissile systems deemed threatening to China.

But for now at least such threats are bluster, said many experts. China is far from taking a dramatic step like abandoning its bedrock nuclear policy, they said.

“I don’t see ‘no first use’ going soon — at least most responsible officers and officials stick to the policy, despite ongoing debate behind the scenes,” said Douglas H. Paal, a China expert who worked on the National Security Council under Presidents Ronald Reagan and George Bush.

Instead, China is likely to respond by spending more on its nuclear, missile and antimissile forces “to ensure survivability of a second-strike force, and expanded penetration aids and decoys to defeat U.S. missile defenses in the event of a second strike,” Mr. Paal said.

In the shorter term, China may accelerate the introduction of a new generation of missiles, the Dongfeng-41, which can be moved around on roads and will also be able to carry multiple warheads, said Mr. Fravel and Ms. Cunningham.

China is also working on a “glide technology to alter the trajectory of a warhead as it nears its target, which could be used to overcome U.S. missile defenses in the long term,” they said.

Adam Wu contributed reporting from Beijing.

A version of this article appears in print on , on Page A8 of the New York edition with the headline: Why U.S. Antimissile System in South Korea Worries China. Order Reprints | Today’s Paper | Subscribe